Power Controller and Control Method for LLC Resonant Converter

ABSTRACT

A power controller for an LLC resonant converter controls a high-side switch and a low-side switch. An ON-time generator in the power controller determines a high-side ON time of the high-side switch and a low-side ON time of the low-side switch in response to the bigger one between a feedback voltage and a burst voltage, where the feedback voltage is generated in response to an output voltage of the LLC resonant converter. A burst-mode controller in the power controller has a triangular-wave generator providing a triangular-wave signal with an amplitude in association with the burst voltage. A comparator comparing the triangular-wave signal and the feedback voltage to determine a break time when both the high-side and low-side switches are turned OFF. The LLC resonant converter operates in a burst mode when the break time is introduced.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of TaiwanApplication Series Number 109110852 filed on Mar. 30, 2020, which isincorporated by reference in its entirety.

BACKGROUND

The present disclosure relates generally to LLC resonant converters, andmore particularly to burst-mode operation controls in LLC resonantconverters that increase power conversion efficiency for light or noloads.

LLC resonant converters are very superior in conversion efficiency amongswitching mode power converters. As known in the art, conduction lossesof power switches, the power losses when power switches conduct current,are some major causes of the power losses in a switching mode powerconverter. Theoretically, an LLC resonant converter can operate its twomain power switches, high-side and low-side switches, to perform zerovoltage switching (ZVS), a technology that a power switch is turned ONaround the moment when the voltage drop across the conduction channel ofthe power switch is about zero. The conduction losses caused by theconductions of high-side and low-side switches could be thereforeminimized. LLC resonant converters are mostly used for high powerapplications, for example, converting power more than 100 W.

An LLC resonant converter suffers, however, in significant switchinglosses when it provides power to a light load or no load. As known inthe art, the switching frequency of an LLC resonant converter increaseswhen its load decreases. Even though the conduction losses of an LLCresonant converter could be suppressed or minimized by ZVS, switchinglosses, the power losses needed to charge or discharge control nodes ofpower switches, inevitably increase along with the increment ofswitching frequency. The significant increment in the switchingfrequency of an LLC resonant converter could cause significant switchinglosses that seriously degrade the power conversion rate of the LLCresonant converter. Accordingly, it is often needed to provide speciallyoperations when the load of an LLC resonant converter is light ornon-existent.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following drawings. In the drawings,like reference numerals refer to like parts throughout the variousfigures unless otherwise specified. These drawings are not necessarilydrawn to scale. Likewise, the relative sizes of elements illustrated bythe drawings may differ from the relative sizes depicted.

The invention can be more fully understood by the subsequent detaileddescription and examples with references made to the accompanyingdrawings, wherein:

FIG. 1 demonstrates LLC resonant converter 100 according to embodimentsof the invention;

FIG. 2 illustrates power controller 102 in FIG. 1;

FIG. 3 demonstrates the waveforms of triangular-wave signal V_(TRI),feedback voltage V_(FB) minus 0.2V, stop signal GATE-STOP, signal S_(H)controlling high-side switch HS, and signal S_(L) controlling low-sideswitch LS, when feedback voltage V_(FB) is below burst voltage V_(BST);

FIG. 4A demonstrates the relationship between feedback voltage V_(FB)and high-side ON time T_(H-ON) and/or low-side ON time T_(L-ON); and

FIG. 4B the relationship between feedback voltage V_(FB) and work dutyDWK.

DETAILED DESCRIPTION

According to embodiments of the invention, an LLC resonant converteroperates in a burst mode when its load is light or non-existent. Whenoperating in a burst mode, break time BRK, when high-side and low-sideswitches constantly turn OFF, alternates with work time WK, whenhigh-side and low-side switches switch periodically and complementarily.The combination of a period of bread time BRK and a period of work timeWK forms a predetermined burst cycle time TBST, which according to oneembodiment of the invention is a constant independent from the load ofan LLC resonant converter.

FIG. 1 demonstrates LLC resonant converter 100 according to embodimentsof the invention, converting input voltage V_(IN) in a primary side intooutput voltage V_(OUT) in a secondary side. Shown in FIG. 1, LLCresonant converter 100 includes power controller 102, high-side switchHS, low-side switch LS, transformer TF, capacitors CIN, CVCC, CL, andCOUT, diodes D1, D2H, and D2L, error amplifier ER, and photo couplerOPT. Inside transformer TF, there are primary winding LP, auxiliarywinding LA, secondary windings LSH and LSL, inductively coupled to eachother. FIG. 1 also shows that transformer TF includes parasitic inductorLPP, representing the leakage inductance and connected in series withprimary winding LP and capacitor CL between connection node N1 and inputground. Parasitic inductor LPP, primary winding LP and capacitor CL forman LLC resonant tank that is driven to resonate and transfer power fromthe primary side to the secondary side.

Power controller 102 according to embodiments of the invention is apackaged integrated circuit having, but not limited to have, powersource pin VCC, feedback pin FB, setting pin BSTS, high-side drive pinHGATE, high-side ground pin HGND, low-side drive pin LGATE, and groundpin GND. Via high-side drive pin HGATE and low-side drive pin LGATE,power controller 102 controls high-side ON time T_(H-ON) of high-sideswitch HS and low-side ON time T_(L-ON) of low-side switch LSrespectively, so as to energize or deenergize the LLC resonant tank.When current and voltage resonate within the LLC resonant tank, diodesD2H and D2L rectify the induced current through secondary windings LSHand LSL to build up output voltage V_(OUT) powering load 104. In themeantime, diode D1 rectifies induced current through auxiliary windingLA to build up operating power voltage V_(CC), which substantiallysupplies power that power controller 102 needs.

Regulation of output voltage V_(OUT) is controlled by the combination oferror amplifier ER and photo coupler OPT, sending feedback throughfeedback pin FB to power controller 102. Feedback voltage V_(FB) atfeedback pin FB is generated in response to the comparison betweenoutput voltage V_(OUT) and predetermined target voltage V_(TRGT). Thepurpose of regulation is to stabilize output voltage V_(OUT) at abouttarget voltage V_(TRGT). For instance, when output voltage V_(OUT)exceeds target voltage V_(TRGT), photo coupler OPT pulls down feedbackvoltage V_(FB) at feedback pin FB, high-side ON time T_(H-ON) andlow-side ON time T_(L-ON) in the next switching cycle shortens, thepower feeding into the LLC resonant tank in the next switching cyclereduces, and the power transferred to the secondary side decreases as aresult, so output voltage V_(OUT) tends to go down to approach targetvoltage V_(TRGT). From another point of view, if output voltage V_(OUT)is about the same as target voltage V_(TRGT), the lighter load 104 thelower feedback voltage V_(FB).

Setting pin BSTS of power controller 102 is electrically connected to anexternal resistor RST, whose resistance for example is detected by powercontroller 102 to determine burst voltage V_(BST). Burst voltageV_(BST), according to embodiments of the invention, could determinehigh-side ON time T_(H-ON) and low-side ON time T_(L-ON) when load 104is light or non-existent.

FIG. 2 illustrates power controller 102, including ON-time generator110, burst-mode controller 112, burst-voltage setting circuit 113,comparator 114, logic 116, and gate drivers 118 and 120.

Burst-voltage setting circuit 113 electrically connects to setting pinBSTS, and detects the resistance of external resistor RST to generateburst voltage V_(BST). For example, burst-voltage setting circuit 113has constant current source IS supplying a constant current flowingthrough external resistor RST, and burst voltage V_(BST) is, but is notlimited to be, the voltage at setting pin BSTS plus 0.2V, which is theminimum value of feedback voltage V_(FB). In one embodiment of theinvention, photo coupler OPT can pull down feedback voltage V_(FB) aslow as 0.2V at most.

Comparator 114 compares feedback voltage V_(FB) with burst voltageV_(BST). According to embodiments of the invention, when feedbackvoltage V_(FB) is below burst voltage V_(BST), comparator 114 outputslow-load signal S_(LOW) with logic “1”; in the opposite, when feedbackvoltage V_(FB) exceeds burst voltage V_(BST), comparator 114 outputslow-load signal S_(LOW) with logic “0”. The configuration of comparator114 and multiplexer 122 is to make multiplexer 122 output the bigger onebetween feedback voltage V_(FB) and burst voltage V_(BST). Simplyspeaking, when feedback voltage V_(FB) is below burst voltage V_(BST),multiplexer 122 outputs burst voltage V_(BST), and power controller 102operates LLC resonant converter 100 in a burst mode, where break timeBRK and alternates with work time WK. When feedback voltage V_(FB)exceeds burst voltage V_(BST), multiplexer 122 outputs feedback voltageV_(FB), and power controller 102 operates LLC resonant converter 100 ina non-burst mode, where only work time WK exists and break time BRKdisappears.

When operating in a non-burst mode, ON-time generator 110 determineshigh-side ON time T_(H-ON) of high-side switch HS and low-side ON timeT_(L-ON) of low-side switch LS in light of feedback voltage V_(FB).ON-time generator 110 includes multiplexer 122, comparators 124 and 126,and ramp generator 128. Multiplexer 122, in response to the output ofcomparator 114, forwards feedback voltage V_(FB) to comparators 124 and126 when operating in a non-burst mode. In the other hand, whenoperating in a burst mode, multiplexer 122 forwards burst voltageV_(BST) to comparators 124 and 126. For example, ramp generator 128starts increasing ramp signal V_(RAMP-H) at the same time when high-sideswitch HS is just turned ON. At the moment when ramp signal V_(RAMP-H)exceeds the signal at the inverted input of comparator 124, which iseither feedback voltage V_(FB) or burst voltage V_(BST), depending onthe comparison between them, output NH of comparator 124 provides arising edge, which resets SR flipflop 130 to turn OFF, via gate driver118, high-side switch HS, so high-side ON time T_(H-ON) is ended.Accordingly, ON-time generator 110 controls the length of high-side ONtime T_(H-ON) based on feedback voltage V_(FB) or burst voltage V_(BST).Analogously, ON-time generator 110 controls the length of low-side ONtime T_(L-ON) based on feedback voltage V_(FB) or burst voltage V_(BST).

Logic 116 is configured to prevent short through from happening, whereshort through refers to a short circuit connected between input voltageV_(IN) and input ground when high-side switch HS and low-side switch LSare turned ON at the same time. When signal S_(H) turns from “1” into“0” for example and gate driver 118 in response starts turning OFFhigh-side switch HS, the falling edge of signal S_(H) makes pulsegenerator 136 generate a pulse to set SR flipflop 132, turning signalS_(L) from “0” into “1”, so gate driver 120 starts turning ON low-sideswitch LS. Accordingly, low-side switch LS is allowed to turn ON onlywhen high-side switch HS turns OFF. Similarly, high-side switch HS isallowed to be turned ON only when low-side switch LS turns OFF. In otherwords, logic 116 is capable of making high-side and low-side switches HSand LS switch periodically and complementarily.

Burst-mode controller 112 has counter 140, digital-to-analog converter142, comparator 144, pulse generators 146 and 150, AND gate 148, and SRflipflop 152. Based on clock CLK counter 140 provides count SD havingthe most significant bit MSB. The most significant bit MSB is the bitpositioned in a binary number having the greatest value.

Clock CLK according to embodiments of the invention could be signalS_(L) or signal S_(H), or any signal generated from an independent clockgenerator. For example, clock CLK could be provided by ramp generator128, which also generates ramp signals V_(RAMP-L) and V_(RAMP-H)periodically. In one embodiment of the invention, count SD changes andcycles from 0 to 31 every predetermined burst cycle time TBST, which is,but is not limited to be, 1/400 sec. Digital-to-analog converter 142provides triangular-wave signal V_(TRI) in response to count SD andburst voltage V_(BST), where triangular-wave signal V_(TRI) could be asaw waveform periodically varying from 0V to burst voltage V_(BST) minus0.2V. The amplitude of triangular-wave signal V_(TRI) is therefore inassociation with burst voltage V_(BST). For example, the value oftriangular-wave signal V_(TRI) could be, according to embodiments of theinvention, expressed as SD*(V_(BST)−0.2)/32. Comparator 144 comparestriangular-wave signal V_(TRI) with feedback voltage V_(FB) minus 0.2V.Supposed that triangular-wave signal V_(TRI-M) is equal totriangular-wave signal V_(TRI) plus 0.2V, comparator 144 equivalentlycompares triangular-wave signal V_(TRI-M) with feedback voltage V_(FB).At the time when feedback voltage V_(FB) is below triangular-wave signalV_(TRI-M), comparator 144 sets, via AND gate 148, SR flipflop 152,making stop signal GATE-STOP “1” in logic and causing both high-sideswitch HS and low-side switch LS OFF. Shown in FIG. 2, when feedbackvoltage V_(FB) minus 0.2V is less than triangular-wave signal V_(TRI),both high-side switch HS and low-side switch LS are constantly OFF; whenfeedback voltage V_(FB) minus 0.2V is more than triangular-wave signalV_(TRI), high-side and low-side switches HS and LS switch periodicallyand complementarily.

Pulse generator 146, configured to be triggered by the falling edge ofsignal S_(L), provides a short pulse to AND gate 148, to open a windowduring which stop signal GATE-STOP is allowed to turn from “0” to “1”.From an aspect of view, the comparison between triangular-wave signalV_(TRI) and feedback voltage V_(FB) minus 0.2V can be recorded by SRflipflop 152 only during the window after low-side switch LS is turnedOFF. Further from another aspect of view, burst-mode controller 112checks, only during that window after low-side switch LS is turned OFF,whether or not triangular-wave signal V_(TRI) exceeds feedback voltageV_(FB) minus 0.2V. Therefore, during the time when high-side switch HSor low-side switch LS is still ON, high-side switch HS or low-sideswitch LS will not be suddenly turned OFF by comparator 144 even iftriangular-wave signal V_(TRI) exceeds feedback voltage V_(FB) minus0.2V.

Pulse generator 150, triggered by the falling edge of the mostsignificant bit MSB, provides a short pulse to reset SR flipflop 152,forcing stop signal GATE-STOP to be “0” in logic, so as to let high-sideand low-side switches HS and LS switch periodically and complementarily.Therefore, no matter how much feedback voltage V_(FB) is, the fallingedge of the most significant bit MSB will cause high-side switch HS andlow-side switch LS to turn ON at least once. It can be understood fromthe configuration of burst-mode controller 112, since the falling edgeof the most significant bit MSB appears at least once everypredetermined burst cycle time TBST, high-side switch HS and low-sideswitch LS turn ON at least once every predetermined burst cycle timeTBST. According to another embodiment of the invention, it is the risingedge of the most significant bit MSB that triggers pulse generator 150to provide a short pulse resetting SR flipflop 152.

FIG. 3 demonstrates the waveforms of triangular-wave signal V_(TRI),feedback voltage V_(FB) minus 0.2V, stop signal GATE-STOP, signal S_(H)controlling high-side switch HS, and signal S_(L) controlling low-sideswitch LS, when feedback voltage V_(FB) is below burst voltage V_(BST).

As shown in FIG. 3, triangular-wave signal V_(TRI) cycles once everypredetermined burst cycle time TBST. Every time when the waveform oftriangular-wave signal V_(TRI) goes across that of feedback voltageV_(FB) minus 0.2V, stop signal GATE-STOP changes its logic value into“0” or “1”. At moment t0, the most significant bi MSB has a fallingedge, so high-side and low-side switches HS and LS start switchingperiodically and complementarily. Moment t0 is also the beginning of apredetermined burst cycle time TBST, as shown in FIG. 3.

Feedback voltage V_(FB) minus 0.2V is higher than triangular-wave signalV_(TRI) during the period from moment t0 to moment t1, so stop signalGATE-STOP is “0”, and high-side and low-side switches HS and LS switchperiodically and complementarily. Therefore, the period from moment t0to moment t1 is referred to as work time WK, during which high-side andlow-side switches HS and LS are controlled to energize the LLC resonanttank. Both high-side ON time T_(H-ON) and low-side ON time T_(L-ON)during work time WK in FIG. 3 are constant, independent from feedbackvoltage V_(FB), because multiplexer 122 in ON-time generator 110 iscurrently providing burst voltage V_(BST) to comparators 124 and 126.

During break time BRK, referring to the period of time from moment t1 tomoment t2, feedback voltage V_(FB) minus 0.2V is below triangular-wavesignal V_(TRI), so stop signal GATE-STOP is “1” in logic, and high-sideand low-side switches HS and LS are constantly OFF. At moment t2, apredetermined burst cycle time TBST ends and a next predetermined burstcycle time TBST starts.

Work duty DWK refers to the ratio of work time WK to predetermined burstcycle time TBST. It is comprehensible from FIG. 3 that, if feedbackvoltage V_(FB) increases, then both work time WK and work duty DWKincrease but the predetermined burst cycle time TBST remains unchanged.

FIG. 4A demonstrates the relationship between feedback voltage V_(FB)and high-side ON time T_(H-ON) and/or low-side ON time T_(L-ON), andFIG. 4B the relationship between feedback voltage V_(FB) and work dutyDWK.

FIG. 4A shows that when feedback voltage V_(FB) exceeds burst voltageV_(BST) each of high-side ON time T_(H-ON) and low-side ON time T_(L-ON)is in positive linear correlation with feedback voltage V_(FB).Nevertheless, when feedback voltage V_(FB) drops to be less than burstvoltage V_(BST), each of high-side ON time T_(H-ON) and low-side ON timeT_(L-ON) depends on feedback voltage V_(FB) no more, and becomes aconstant determined by burst voltage V_(BST).

FIG. 4B shows that if feedback voltage V_(FB) exceeds burst voltageV_(BST) work duty DWK is always 100%, meaning that high-side andlow-side switches HS and LS switch periodically and complementarily allthe time. Nevertheless, if feedback voltage V_(FB) drops to be less thanburst voltage V_(BST), work duty DWK becomes less than 100% because ofthe occurrence of break time BRK, and the less feedback voltage V_(FB)the less work duty DWK.

According to embodiments of the invention, when the load is light ornonexistent, LLC resonant converter 100 is capable of being operated ina burst mode, in which break time BRK alternates with work time WK. Theintroduction of break time can reduce switching losses of high-side andlow-side switches HS and LS, improving power conversion efficiency.

The predetermined burst cycle time TBST could be properly set to avoiduncomfortable audible noise that probably occurs when LLC resonantconverter 100 is operated in a burst mode when the load is light.

A system engineer could select external resistor RST to determine theload threshold which determines the quantity of load 104 for LLCresonant converter 100 to operate in a burst mode. External resistor RSTalso can determine high-side ON time T_(H-ON) and low-side ON timeT_(L-ON), which are two constants when LLC resonant converter 100operates in a burst mode.

According to embodiments of the invention, each of high-side switch HSand low-side switch LS is turned ON at least once every predeterminedburst cycle time TBST, which is internally set by burst-mode controller112 and consists of one break time BRK and one work time WK. Withoutmaking each of high-side switch HS and low-side switch LS turned ON atleast once every predetermined burst cycle time TBST, break time BRKmight last so long that the real predetermined burst cycle time TBSTcould be an integer multiple of the predetermined burst cycle time TBSTinternally set by burst-mode controller 112. It might induceuncomfortable audible noise if the real predetermined burst cycle timeTBST is too long.

While the invention has been described by way of example and in terms ofpreferred embodiment, it is to be understood that the invention is notlimited thereto. To the contrary, it is intended to cover variousmodifications and similar arrangements (as would be apparent to thoseskilled in the art). Therefore, the scope of the appended claims shouldbe accorded the broadest interpretation so as to encompass all suchmodifications and similar arrangements.

What is claimed is:
 1. A power controller for controlling a high-sideswitch and a low-side switch, comprising: an ON-time generator,controlling a high-side ON time of the high-side switch and a low-sideON time of the low-side switch in response to a feedback voltage,wherein the feedback voltage is generated in response to an outputvoltage of a power converter; a burst-mode controller, comprising: atriangular-wave generator, providing a triangular-wave signal with apredetermined burst cycle time, wherein an amplitude of thetriangular-wave signal is in association with a burst voltage; and afirst comparator comparing the triangular-wave signal and the feedbackvoltage, wherein the first comparator causes the high-side switch andthe low-side switch constantly turned OFF when the feedback voltage isbelow the triangular-wave signal; and a second comparator for comparingthe burst voltage and the feedback voltage, configured to cause thelow-side and the high-side ON times controlled in response to the burstvoltage when the feedback voltage is below the burst voltage.
 2. Thepower controller as claimed in claim 1, wherein the triangular-wavesignal varies between a predetermined voltage and the burst voltage. 3.The power controller as claimed in claim 2, wherein the triangular-wavegenerator comprises: a counter providing a count in response to a clock;and a digital-to-analog converter for generating the triangular-wavesignal in response to the count and the burst voltage.
 4. The powercontroller as claimed in claim 3, wherein the count has a mostsignificant bit (MSB) and the burst-mode controller is configured tomake the high-side and low-side switches turned ON in response to achange of the MSB.
 5. The power controller as claimed in claim 1,wherein the burst-mode controller is configured to make the high-sideand low-side switches turned ON at least once every predetermined burstcycle time.
 6. The power controller as claimed in claim 1, wherein thepower controller includes a setting circuit connected to a resistor togenerate the burst voltage.
 7. The power controller as claimed in claim6, wherein the power controller is in a form of a packaged integratedcircuit with a setting pin, and the resistor is connected to the settingcircuit via the setting pin.
 8. A control method for an LLC resonantconverter with high-side and low-side switches, the control methodcomprising: providing a feedback voltage in response to an outputvoltage of the LLC resonant converter; providing a burst voltage;generating a triangular-wave signal with a predetermined burst cycletime based on the burst voltage; comparing the triangular-wave signalwith the feedback voltage; turning OFF the high-side and low-sideswitches if the feedback voltage is below the triangular-wave signal;and controlling a high-side ON time of the high-side switch and low-sideswitch and a low-side ON time of the low-side switch in response to thebigger one between the burst voltage and the feedback voltage.
 9. Thecontrol method as claimed in claim 8, wherein the triangular-wave signalvaries between a predetermined voltage and the burst voltage.
 10. Thecontrol method as claimed in claim 8, wherein the LLC resonant converterhas a power controller and a resistor, and the control method used bythe power controller comprises: providing the burst voltage in responseto the resistor.
 11. The control method as claimed in claim 8,comprising: turning ON the high-side switch and the low-side switch atleast once every predetermined burst cycle time.
 12. The control methodas claimed in claim 8, comprising: providing a count in response to aclock; and generating the triangular-wave signal in response to thecount and the burst voltage.
 13. The control method as claimed in claim12, wherein the count has a most significant bit, and the control methodcomprises: turning ON the high-side switch and the low-side switch atleast once after the most significant bit changes.
 14. An LLC resonantconverter, comprising: a high-side switch and a low-side switchelectrically connected to each other via a connection node and betweentwo power lines; a resonant tank electrically connected to theconnection node; a secondary winding inductively coupled to the resonanttank to generate an output voltage; and a power controller forcontrolling the high-side switch and the low-side switch, wherein thepower controller is configured to perform the following stepscomprising: providing a feedback voltage in response to the outputvoltage; controlling a high-side ON time of the high-side switch and alow-side ON time of the low-side switch in response to the bigger onebetween a burst voltage and the feedback voltage; generating atriangular-wave signal with a predetermined burst cycle time based onthe burst voltage; and determining, in response to a comparison betweenthe triangular-wave signal and the feedback voltage, a break time whenthe high-side switch and the low-side switch are constantly turned OFF.15. The LLC resonant converter as claimed in claim 14, wherein the powercontroller comprises: a burst-mode controller, comprising: atriangular-wave generator providing the triangular-wave signal with anamplitude in association with the burst voltage; and a first comparatorcomparing the triangular-wave signal and the feedback voltage, whereinthe first comparator causes the high-side switch and the low-side switchconstantly turned OFF to start the break time when the feedback voltageis below the triangular-wave signal.
 16. The LLC resonant converter asclaimed in claim 14, wherein the power controller comprises: an ON-timegenerator, controlling the high-side ON time and the low-side ON time inresponse to a feedback voltage; and a comparator for comparing the burstvoltage and the feedback voltage, configured to affect the ON-timegenerator and cause the low-side and the high-side ON times controlledin response to the burst voltage when the feedback voltage is below theburst voltage.
 17. The LLC resonant converter as claimed in claim 14,wherein the power controller turns ON the high-side switch and thelow-side switch at least once every predetermined burst cycle time. 18.The LLC resonant converter as claimed in claim 17, wherein the powercontroller comprises: a counter providing a count in response to aclock; and a digital-to-analog converter for generating thetriangular-wave signal in response to the count and the burst voltage.19. The LLC resonant converter as claimed in claim 18, wherein the counthas a most significant bit (MSB) and the power controller is configuredto make the high-side and low-side switches turned ON in response to achange of the MSB.
 20. The LLC resonant converter as claimed in claim14, further comprising a resistor electrically connected to the powercontroller, and the power controller generate the burst voltage inresponse to the resistance of the resistor.